Rab3A is a low-molecular weight GTP-binding protein of synaptic vesicles that regulates synaptic vesicle exocytosis and undergoes a GTP hydrolysis/GTP-GDP exchange cycle and membrane dissociation-association cycle as a function of exocytosis. Rab3A binds to another synaptic vesicle protein, rabphilin in a GTP-dependent way. Rabphilin is composed of three principal domains: an N-terminal cysteine-rich region, which is highly conserved throughout evolution; a middle region that is substrate to multiple protein kinases; and a C-terminal region containing two C2-domains. We are currently investigating the interaction of Rab3A-GTP with its rabphilin target domain, which consists out of the minimal rabphilin binding-fragment 40-170. Using a co-expression strategy we have been able to produce a stable complex of Rab3A-GTP and the Zn-binding domain 40-170 of rabphilin. We recently crystallized this complex. The monoclinic crystals belong to space group C2 and show diffraction up to 2.8 E on a rotating anode as X-ray source. We were also able to grow diffracting (dmin ~ 2.6 E) selenomethionine containing crystals. Initial interpretation of isomorphous difference Patterson maps revealed a significant solution for the 12 selenium sites.